1. Field of the Invention
The invention relates to a compound particularly useful as a writing monomer in photopolymer formulations. The invention further relates to a photopolymer formulation comprising at least a polyol component, a polyisocyanate component, a writing monomer and a photoinitiator and also to the use of the photopolymer formulation for producing holographic media.
2. Description of Related Art
The uses for photopolymer formulations are decisively determined by the refractive index contrast Δn produced in the photopolymer by holographic exposure. In holographic exposure, the interference field of signal light beam and reference light beam (in the simplest case, that of two plane waves) is mapped into a refractive index grating by the local photopolymerization of, for example, high refractive index acrylates at loci of high intensity in the interference field. The refractive index grating in the photopolymer (the hologram) contains all the information of the signal light beam. Illuminating the hologram with the reference light beam only will then reconstruct the signal. The strength of the signal thus reconstructed relative to the strength of the incident reference light is called diffraction efficiency, DE in what follows.
In the simplest case of a hologram resulting from the superposition of two plane waves, the DE is the ratio of the intensity of the light diffracted on reconstruction to the sum total of the intensities of the incident reference light and the diffracted light. The higher the DE, the greater the efficiency of a hologram with regard to the amount of reference light needed to visualize the signal with a fixed brightness.
High refractive index acrylates are capable of producing refractive index gratings of high amplitude between regions of low refractive index and regions of high refractive index and hence of producing holograms of high DE and high Δn in photopolymer formulations. It must be noted that DE is dependent on the product of Δn and the photopolymer layer thickness d. The width of the angular range in which the hologram is visualized (reconstructed) on monochromatic illumination, for example, is solely dependent on the layer thickness d.
When the hologram is illuminated with white light, for example, the width of the spectral range which can contribute to reconstructing the hologram is likewise only dependent on the layer thickness d. The relationship is that the smaller the d, the greater the particular acceptance widths. Therefore, to produce bright and easily visible holograms, it is generally desirable to seek a high Δn and a low thickness d while maximizing DE. That is, increasing Δn increases the latitude to engineer the layer thickness d without loss of DE for bright holograms. Therefore, the optimization of Δn is of outstanding importance in the optimization of photopolymer formulations (P. Hariharan, Optical Holography, 2nd Edition, Cambridge University Press, 1996).
WO 2008/125229 A1 discloses photopolymer formulations comprising mono- and difunctional writing monomers of high molecular weight. The media comprising these formulations make it possible to write reflection holograms that are very useful for data storage for example. However, producing and processing the formulations presents problems: the writing monomers comprised therein have a high viscosity and/or high TG values (TG=glass transition temperature). This makes it difficult to achieve a uniform distribution of the writing monomers in the photopolymer formulation and a medium produced therefrom. In addition, when the known formulations are used, writing monomer agglomerates can form in the polymer matrix and appreciably compromise the quality of the media and/or of the holograms written therein. In such cases, the holographic materials become hazy.
WO 2008/125199 describes trifunctional urethane acrylate writing monomers and photopolymer formulations comprising same. They make it possible to achieve a particularly high refractive index contrast when the formulations additionally contain specific fluorourethanes. Such fluorourethanes and their use in photopolymer formulations are described for example in the as yet unpublished European application of application number are EP 09013770.4.
In principle, with these formulations, the refractive index contrast increases with the fluorourethane content. However, as the fluorourethane content increases, the optical quality becomes impaired by haze.
Transmission holograms are a particular form of holograms in that the reference beam and the object beam irradiate the holographic medium from the same side to produce the holograms. Transmission holograms have various applications. Light guidance is to be mentioned in particular here as a diffractive optical element. Such optical element can be used in demanding applications such as spectroscopy or astronomy. They are similarly suitable for use in electronic 3D displays.
Owing to the geometry of the object and signal beams which are made to interfere, the grating spacing in transmission holograms is large compared with reflection holograms. Depending on the wavelength, the grating spacing can be between 500-1000 nm. Since the mechanism of hologram formation in the photopolymer formulations is based on the diffusion of the writing monomers, it is difficult to develop writing monomers that are able to diffuse sufficiently far in the case of the large grating spacing customary for transmission holograms. Yet this is a prerequisite to achieve a high refractive index contrast (Δn). The photopolymers known from the area of reflection holograms are frequently not suitable for this in that they do not lead to a sufficiently high refractive index contrast.